1. Field of the Invention
This invention relates to an image heating apparatus such as a fixing apparatus or an apparatus for improving the surface property of an image carried on an image forming apparatus such as a copier or a printer.
2. Related Art
Apparatuses of the heat roller type and the film heating type have heretofore been widely used as fixing apparatuses used in image forming apparatuses of the electrophotographic type, the electrostatic recording type, etc. Particularly, a method of minimizing electric power consumption to the utmost without supplying electric power to a fixing apparatus during standby, and more particularly a heating and fixing method by a film heating system of heating and pressurizing a recording material while passing the recording material to a nip area formed by a pressure member being in pressure contact with a heating member with a film member interposed therebetween to thereby fix a toner image on the recording material as an unfixed image on the recording material are proposed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075, Japanese Patent Application Laid-Open No. 4-204980, etc.
FIG. 10 of the accompanying drawings schematically shows the construction of the essential portions of an example of the fixing apparatus adopting the film heating process.
Such a fixing apparatus, as shown in FIG. 10, has a heater 111 which is a heating member fixedly supported by a stay holder (supporting body) 112, fixing film 113 which is a thin and heat-resistant film member, and an elastic pressure roller 120 which is a pressure member brought into pressure contact with the heater 111 with the film member 113 interposed therebetween to thereby form a nip area (hereinafter referred to as the fixing nip portion) N of a predetermined nip width.
The fixing film 113 is a cylindrically shaped or endless-belt-shaped or rolled web-shaped member conveyed in the direction of arrow a by driving means (not shown) or the rotational force of the pressure roller 120 while being in close contact with the surface of the heater 111 in the fixing nip portion N.
The heater 111 receives the supply of electric power from a power source (not shown) and generates heat and is controlled to a predetermined temperature.
When in a state in which the heater 111 has been heated and controlled to the predetermined temperature and the fixing film 113 has been conveyed in the direction of arrow, a recording material P bearing an unfixed toner image t thereon as a material to be heated is introduced between the fixing film 113 in the fixing nip portion N and the pressure roller 120, the recording material P comes into close contact with the surface of the fixing film 113 and is nipped and conveyed by the fixing nip portion N with the fixing film 113. In this fixing nip portion N, the recording material P and the toner image t are heated by the heater 111 through the fixing film 113 and the toner image t on the recording material P is fixed. That portion of the recording material P which has passed through the fixing nip portion N is peeled off from the surface of the fixing film 113 and is conveyed.
A ceramic heater is generally used as the heater 111 as a heating member. For example, the heater 111 comprises a substrate 111a made of ceramics having electrically insulativeness, good heat conductivity and low heat capacity such as alumina, and a heat-generating resistance layer 111b of silver palladium (Ag/Pb), Ta2N or the like formed on the surface (the surface facing the fixing film 13) of the substrate 111a along the lengthwise direction (a direction perpendicular to the conveying direction of the recording material P) of the substrate 111a as by screen printing, that surface of the substrate 111a on which the heat-generating resistance layer 111b is formed being covered with a thin glass protective layer 111c. 
This heater 111 which is a ceramic heater is such that by electric power being supplied to the heat-generating resistance layer 111b, the heat-generating resistance layer 111b generates heat and heat the substrate 111a made of ceramics and the glass protective layer 111c and the entire heater 111 rapidly rises in temperature. This temperature rise of the heater 111 is detected by temperature detecting means 114 disposed on the back of the heater 111 and is fed back to a power supply control portion (not shown) which is control means. The power supply control portion controls the electric power supplied to the heat-generating resistance layer 111b so that the temperature of the heater 111 detected by the temperature detecting means 114 may be maintained at a predetermined substantially constant temperature (fixing temperature). In this manner, the heater 111 is heated and controlled to the predetermined fixing temperature.
The fixing film 113 has its thickness formed considerably small, e.g. to 20 to 70 xcexcm, in order to efficiently give the heat of the heater 111 to the recording material P as the material to be heated in the fixing nip portion N. This fixing film 113 is formed by three layers, i.e., a film base layer, a primer layer and a releasing property layer, and the film base layer side is the heater 111 side and the releasing property layer side is the pressure roller 120 side. The film base layer is formed of polyimide, polyamideimide, PEEK or the like higher in insulativeness than the glass protective layer 111c of the heater 111, and has heat resistance and high elasticity. Also, the mechanical strength such as the tearing strength of the entire fixing film 113 is kept by the film base layer. The above-mentioned primer layer is formed by a thin layer having a thickness of the order of 2 to 6 xcexcm. The above-mentioned releasing property layer is a toner offset preventing layer for the fixing film 113, and is formed by coating the primer layer with fluorine resin such as PFA, PTFE or FEP to a thickness of the order of 10 xcexcm.
Also, the stay holder 112 is formed, for example, by a member made of heat-resistant plastic, and holds the heater 111 and serves also as the conveyance guide of the fixing film 113.
In a heating apparatus of the film heating type using such thin fixing film 113, due to the high rigidity of the heater 111 made of ceramics, the pressure roller 120 having an elastic layer 122 becomes flat in the pressure contact portion thereof, following the flat underside of the heater 111 with which it is brought into pressure contact, and formes the fixing nip portion N of a predetermined width, and only the fixing nip portion N is heated to thereby realize heating and fixing of quick start.
In the fixing apparatus of the above-described construction, the disposition relation between the heat-generating resistance layer 111b of the heater 111 and the pressure roller 120 will now be described with reference to FIG. 11 of the accompanying drawings.
As shown in FIG. 11, the width W of the heat-generating resistance layer 111b of the heater 111 in the longitudinal direction thereof is somewhat narrow as compared with the width D of the elastic layer 122 of the pressure roller 120 brought into contact therewith with the fixing film 113 interposed therebetween in the same direction. This is for preventing the heat-generating resistance layer 111b from protruding from the pressure roller 120 in the same direction to thereby locally rise in temperature and be damaged by the thermal stress thereof. Also, the heat-generating resistance layer 111b is formed with a width sufficiently wider than the sheet passing area of the recording material P bearing the toner image t thereon. Thereby, the temperature fall of the end portions (due to the leakage of the heat to electrical contacts for power supply and connectors in the lengthwisely end portions of the heater 111) can be eliminated, whereby a good fixing property is obtained over the whole surface of the recording material P. Further, there is a case where the width of the sheet passing area end portions of the heat-generating resistance layer 111b is narrowed down and the amount of heat generation in the end portions is increased to thereby make up for the fixing property of the end portions.
Thereby, the heat from the heat-generating resistance layer 111b of the heater 111 is given to the recording material P conveyed between the fixing film 113 and the pressure roller 120, and acts to fuse and fix the toner image t on the recording material P.
Also, the present example is a center standard apparatus in which a recording material conveyance standard S is provided at the lengthwise center of the recording material passing area of the main body of an image forming apparatus.
Further, as shown in FIG. 11, temperature detecting means 114 such as a thermistor and a thermoprotector 115 such as a temperature fuse or a thermoswitch for shutting down the supply of electric power to the heat-generating resistance layer 111b of the heater 111 during speeding are brought into contact with the back of the heater 111, and these are disposed in the conveyance area of a recording material P of a definite size having a minimum width (within the minimum sheet passing width) which can be conveyed by the image forming apparatus.
The temperature detecting means 114 is provided within the minimum sheet passing width in order to heat and fix the toner image t on the recording material P at a moderate fixing temperature without causing a problem such as bad fixing or high temperature offset even when a recording material P of a minimum width which can be conveyed by the main body of the image forming apparatus is conveyed. On the other hand, the thermoprotector 115 is also provided within the minimum sheet passing width in order not to cause, in the non-sheet-passing area when the recording material P of the minimum width is conveyed, the problem that the recording material is overheated in the non-sheet-passing area smaller in heat resistance than the sheet-passing area, whereby even during ordinary conveyance, the thermoprotector 115 malfunctions and the power supply is shut out.
Now, the thermoprotector 115 is brought into contact with the back of the heater 111, whereby the amount of heat generated by the heat-generating resistance layer 111b is taken away by the thermoprotector 115 and a sufficient amount of heat is not given to the recording material P, and bad fixing is sometimes caused at the contact position of the thermoprotector 115. In order to prevent this, at the position 111bxe2x80x2 of the heat-generating resistance layer 111b corresponding to the contact position of the thermoprotector 115, as shown in FIG. 11, the width of a portion of the heat-generating resistance layer 111b of the heater 111 is somewhat narrowed and the resistance value of the above-mentioned contact position is made greater than that of the other portion to thereby secure an amount of heat generation. Thereby the amount of heat supply to the recording material P is made constant over the lengthwise direction and good heating and fixing free of uneven fixing are realized. The temperature detecting means 114 is likewise brought into contact with the back of the heater 111 and therefore, it is feared that the heat generated by the heat-generating resistance layer 111b is likewise taken away by the temperature detecting means 114, but by using temperature detecting means 114 of a small heat capacity such as a chip thermistor, it is possible to make the amount of heat taken away from the heater Ill small. Therefore, even if the countermeasure as described above similar to that for the thermoprotector 115 is not adopted, uniform fixing becomes possible without spoiling the uniformity of fixing of the recording material in the lengthwise direction thereof.
In the above-described conventional fixing apparatus, when recording materials of different sizes (sheet widths) are continuously passed to the nip area, the amount of heat taken away from the heater by the sheet passing differs greatly between the sheet passing portion and the non-sheet-passing portion and therefore as the sheets are passed, the temperature of the non-sheet-passing portion of which the amount of heat is not taken away by the sheets gradually rises (hereinafter referred to as the temperature rise of the non-sheet-passing portion). Therefore, during the passing of a small size sheet, this problem has been coped with by a method of reducing the throughput (number of sheets conveyed per unit time). As the method of reducing the throughput, there is adopted a method of uniformly reducing the throughput for a number of sheet free of any problem even under a condition under which the temperature rise of the non-sheet-passing portion is worst (thick paper of a small size or the like), or a method of providing a temperature detecting member such as a thermistor in the non-sheet-passing portion, and reducing the throughput when it rises to a predetermined temperature.
However, when a great deal of small size sheets are continuously passed to the fixing nip portion, the temperature of the non-sheet-passing portion moves to the downstream side of the fixing nip portion with respect to the sheet passing direction due to the rotation of the pressure roller and a high temperature portion is formed. This high temperature portion expedites the wear of the surface and inner surface of the fixing film, and there may occur the offset image by a reduction in the releasing property of the surface of the fixing film or the bad paper conveyance (such as slippage or jam) by an increase in the sliding resistance of the film.
Also, in order to more positively suppress the temperature rise of the non-sheet-passing portion and improve the capability of continuously fixing small size paper, there has been adopted a method called zone heating which is to provide a plurality of heat generating members corresponding to paper sizes, and change over the heat generating members in conformity with the paper sizes. In this method, in order to secure the fixing property of large size sheets after the passing of small size sheets, there is adopted a construction for warming up the non-sheet-passing portion at a predetermined percentage even during the passing of small size sheets and therefore, when a great deal of small size sheets are passed, there is the possibility that the temperature of the non-sheet-passing portion gradually rises and damage similar to that described above is given to the fixing film and offset images or bad conveyance (such as slip jam) occurs.
The present invention has been made in view of the above-noted problem and an object thereof is to provide an image heating apparatus which can suppress the excessive temperature rise of a non-sheet-passing portion.
Another object of the present invention is to provide an image heating apparatus which can suppress the damage of the apparatus.
Still another object of the present invention is to provide an image heating apparatus comprising:
a heating member;
a first heat generating element mounted on the heating member;
a second heat generating element mounted on the heating member;
a temperature detecting element for detecting the temperature of the heating member, the temperature detecting element being disposed in an area where a recording material of a predetermined minimum size does not pass; and
power supply control means for controlling electric power supply to the first and second heat generating elements in conformity with both of the detected temperature by the temperature detecting element and the number of continuously passing recording materials.
Yet still another object of the present invention is to provide an image heating apparatus comprising:
a heating member;
a first heat generating element mounted on the heating member; and
a second heat generating element mounted on the heating member;
wherein when the temperature of that area of the heating member where a recording material of a predetermined minimum size does not pass is lower than a predetermined temperature, the first and second heat generating elements generate heat, and when the temperature of the area becomes higher than the predetermined temperature and the number of continuously passing recording materials becomes greater than a predetermined number, the first heat generating element continues to generate heat and the second heat generating element stops generating heat.
A further object of the present invention is to provide an image heating apparatus comprising:
a heating member;
a first heat generating element mounted on the heating member; and
a second heat generating element mounted on the heating member, the width of the second heat generating element in the longitudinal direction thereof being substantially equal to that of the first heat generating element;
wherein when the temperature of that area of the heating member where a recording material of a predetermined minimum size does not pass is lower than a predetermined temperature, the first and second heat generating elements generate heat, and when the temperature of the area is higher than the predetermined temperature, the second heat generating element does not generate heat, but the first heat generating element generates heat.
Still a further object of the present invention is to provide an image heating apparatus comprising:
a heating member;
a plurality of heat generating elements mounted on the heating member;
transfer control means for controlling the transfer of recording materials, the transfer control means decreasing the number of sheets conveyed per unit time when the temperature of that area of the heating member where a recording material of a predetermined minimum size does not pass rises; and
power supply control means for controlling electric power supply to the plurality of heat generating elements, the power supply control means decreasing the number of heat generating elements which generate heat when the temperature of the area of the heating member rises.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.